US 3801426 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
United States Patent [191 Putman et al.
[ Apr. 2, 1974 1 DRYER CONTROL AND GRADE CHANGE SYSTEM FOR A PAPER MACHINE  Inventors: Richard E. Putman, Penn Hills;
James T. Carleton, Pittsburgh, both of Pa.
 Assignee: Westinghouse Electric Corporation,
 Filed: Aug. 26, 1970  Appl. No.: 67,180
 US. Cl 162/198, 34/41, 34/48, 73/75, 162/252, 162/263, 235/151.3  Int. Cl D2lf 5/06, G06f 15/46  Field of Search 162/198, 252, 259, 263, 162/258; 34/54, 41, 48;235/151.3, 151.35; 73/75 [5'61" Riihe Cited UNITED STATES PATENTS 3,000,438 9/1961 Alexander 162/259 3,413,192 11/1968 Beecher 162/259 2,922,475 1/1960 Alexander 162/252 3,490,689 l/1970 Hart et a1 162/252 X 3,216,241 11/1965 Hansen 73/75 3,295,842 1/1967 Stelling, Jr. et a1 34/48 X OTHER PUBLICATIONS Perry et al., Computer Control of a Fine Paper Machine .Tappi, Vol. 48 No. 6, (June 1965) p.
Thompsen, The Paper Machine Under Digital Computer Control The Paper Industry (May 1962) p. 72-76, 79.
Primary ExaminerS. Leon Bashore Assistant Examiner-Alfred DAndrea, Jr. Attorney, Agent, or Firm-R. G. Brodahl  ABSTRACT A computer system for controlling a paper sheet drying machine characterized in that a feed-forward signal is developed proportional to desired steam temperature in the machine drying drums from a consideration of the empirical relationship between caliper and production rate. This same of caliper to production rate is used to predict the production rate at a new caliper while maintaining the steam temperature constant. By this means, a grade change transfer path can be computed which avoids the need to change drying conditions. The speed at which the grade can be changed is merely a function of the mechanical response of the machine and completely divorced from the much longer thermal responses. After the new caliper is established, the steam temperature can be varied leisurely over a period of time to increase the production rate.
9 Claims, 7 Drawing Figures 2ND 3RD 4TH DRYER DRYER MOTOR CONTROL 70% STEAM s3 STEAM s2 STEAM 56 v 3 COMPUTER &ifl 36 F he INPUT PANEL PATENTEU APR 2 I974 SHEET 2 0 4 Ch owwam m2 I 042 CA LIPER (POINTS) 3 G C... O O O m M w w T-r b E s W R E w R 0 w R F w 9 T P M A T 4 w M P N 2 N n v M w m w ME L R N m m -mw U 0 l RT T E L, EA T A YR S C RE DP HM E 0 MT r q 0 W O m Q 8 4 F log 29.5-5 log CALIPER 4 mm F 2.0 log CALIPER PAIENTEDAPR 2 IQII 3.801.426
SHEET '4 BF 4 ENTER NEw DEsIRED CALIPERH) AND PRODUCTION RATE D (FI* P A -A 92 i i 90 CALCULATE C S C= (29.55Iog f) w F=K(DW-V) IOO IIO K 70 CHANGE CALIPER (II INCREASE IN Two POINT VELOCITY INCREMENIS UNTIL V I=I 72 SLICE /lO2 III CONTROL CALCULATE NEw J I PRODUCTION RATE -gag? i W T I04 l|4 CALCULATE NEw 56 CALCULATE Y sPEED E3 V=|79O 8W TS3=TS4TO+D(3O") s3 s5 STEAM vALvE ExIT CONTROL 3R Y R 58 2ND DDR E 6 I r us 60 STEAM vALvE CIASIQORL CONTROL 4TH DRYER FIG. 7.
DRYER CONTROL AND GRADE CHANGE SYSTEM FOR A PAPER MACHINE BACKGROUND OF THE INVENTION In the manufacture of paperv from wood pulp, the pulp in a head box passes through a slice gap or slit onto a wire comprising a fine wire screen in the form of an endless belt, usually several feet wide. Vibration of the screen spreads the material into a thin uniform layer or web. As it is carried along the wire, much of the water in the soft mass drains out. To prevent the material from running over the edges of the wire, rubber belts are usually placed on either side. Mechanical suction removes much of the water from the wet pulp mass, after which the web passes between rollers to a thick woolen felt which is carried between a series of heavy rollers. Passing between these, the paper is pressed into a firm, thin sheet; and thereafter passes over a series of heated drying drums.
It has been found that as the moist paper web passes over the aforesaid drying drums, there is a distinct relationship between the caliper or thickness of the sheet and the speed of the machine or its production rate. As the caliper or thickness of the sheet increases, the speed of the machine must be decreased since with increased caliper, the mass and water content of the paper sheet are greater. Hence, the paper must remain in contact with the heated rolls for a longer period of time. The production rate at higher calipers can be increased by increasing the temperature of the steam fed into the drying drums; however when changing from a low caliper to a higher caliper or vice versa, simply changing steam temperature is an unsatisfactory method for control because of the inherent thermal delays involved in heating or cooling the drums.
Furthermore, it has been found that there are limits on the maximumand minimum machine speeds as a function of caliper, and this regardless of the temperature of the drying drums. As a matter of fact, a plot of possible machine speeds versus caliper yields an envelope of operating speeds, the maximum and minimum speeds decreasing as caliper or thickness increases. The reason'for this is not entirely understood; but perhaps the upper speed limit is due to the effect of centrifugal force causing the sheet to leave the rolls at higher speeds, which decreases the drying effect. The lower speed limit is more difficult to understand. One would imagine that the drying would be very efficient at lower speeds; but the fact remains that there is a lower speed and production rate, and this regardless of steam temperature. All of this can be summarized by saying that at any one caliper, the speed of the machine and its production rate can be varied between upper and lower limits; however when the caliper is changed, the upper and lower speed limits change also. While it is theoretically possible to change caliper while maintaining speed constant by varying the temperature of the steam fed to the drying drums, this is an inherently slow process and may result in the production of a substandard product.
SUMMARY OF THE INVENTION In accordance with the present invention, a control system for a paper drying machine is provided wherein an operator initially selects the caliper and production rate at which the machine is to run, the latter parameter being limited by the maximum upper and lower machine speeds as explained above. Assuming that the caliper is at the selected value, a feed-forward signal proportional to desired steam temperature in the drying drums is derived from a consideration of production rate, the temperature of the sheet entering the dryer and the desired temperature of the sheet leaving the dryer. The temperature of the sheet entering the drying machine is maintained constant; while the final parameter to be controlled is the exit temperature since this is an indication of the amount of moisture in the sheet. Accordingly, a feedback signal proportional to actual exit temperature is utilized to modify the feedforward signal and establish the correct steam temperature of the steam entering the drying drums.
Now, if it is desired to change caliper and production rate, a grade change transfer path is computed while maintaining the speed of the machine within the upper and lower maximum permissible limits. This transfer path is computed based on the assumption that the steam temperature to the drying drums, the input temperature of the sheet and the exit temperature of the sheet are all constant. Since there is a definite relationship between these temperatures, caliper, and production rate, a transfer function can be calculated which will move the speed or production rate to a new value as caliper is changed while maintaining the machine operation within the aforesaid upper and lower maximum limits. After the new speed and production rate are reached, the steam temperature to the drying drums can be changed to further increase the production rate, again keeping within the maximum and minimum limits to the equipment.
The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification, and in which:
FIG. 1 is a schematic representation of a paper drying machine, showing itsconnections to the computer control of the present invention;
FIG. 2 is a plot of caliper versus machine speed showing the envelope defining the upper and lower maxi mum permissible speeds;
FIG. 3 is a plot of caliper versus various control parameters including steam per ton of paper, total steam flow and fourth dryer steam temperature;
FIG. 4 is a logarithmic plot of caliper versus a constant based upon the assumption that the steam temperature and the input and output temperatures of the strip are constant;
FIG. 5 is a plot of caliper versus machine speed showing certain transfer paths for changing machine speed while changing caliper;
FIG. 6 is a plot of caliper versus production rate showing the same transfer paths for changing machine production rate to change caliper; and
FIG. 7 is a flow diagram showing the operation of the computer control of the present invention.
With reference now to the drawings, and particularly to FIG. 1, the system shown includes a head box 10 which consists, essentially, of a tank or container usually closed at the top and having an inlet at its bottom for the introduction of fresh pulp. A slice gap 12 is formed between the bottom of the tank 10 and a slice 14 which is a vertically extending plate, the lower edge of which defines the upper edge of the slice gap. This plate can be moved upwardly or downwardly to vary the vertical dimension of the slice gap and, hence, the thickness of the pulp mat fed onto a wire or metallic gauze belt 16. As mentioned above, the pulp, as it issues through the slice gap 12, spreads onto the wire 16 .where the water drains out of the pulp mat, both by gravity and by mechanical suction.
The thickness of the mat formed on the wire 16 is a function not only of the height of the slice. gap but also the ratio of the spouting velocity of the pulp leaving the slice gap to the speed of the wire onto which it is deposited. The spouting velocity, in turn, is a function of the pressure head in the head box.
From the wire 16, the pulp mat or web passes between rollers, not shown, to a thick woolen felt which is carried between a series of rollers, also not shown but located at the schematic representation identified by the reference numeral 18 in FIG. 1. Passing between these heavy rollers, the paper is pressed into a firm thin sheet, and thereafter passes over a series of heated drying rolls.
Actually, there are three sets of drying rolls identified in FIG. 1 as the pre-dryer 20, the second-third dryer 22 and the fourth dryer 24. Each dryer consists of drums 26 which are hollow and supplied interiorly with steam to heat them. Between the dryers and 22 is a press 28; and, likewise, between the dryers 22 and 24 is a press 30 which further compresses the pulp mat or web during the drying process. At the exit end of the fourth dryer 24 is a sizing press 32. g
The main function of the pre-dryer 20 is to heat the pulp web to a predetermined temperature. It has been .found from actual operating experience that if the web,
upon leaving the pre-dryer, is at a certain constant temperature, then the remaining dryers can be operated much more efficiently. There is no theoretical justification for this; however it is a fact and is something that must be found out from experience. Means, not shown herein, are provided for maintaining the temperature at the output of the pre-dryer constant.
The entire drying process is controlled by a computer, generally indicated by the reference numeral 34 in FIG. 1, this computer being provided with a resident program 36, hereinafter described. The computer 34 also has an input panel 38 under the control of an operator wherein signals F proportional to desired production rate in tons per hour and t* proportional to desired caliper are fed to the computer 34. Also applied to the computer 34 is a signal T on lead 40 proportional to the temperature of the web at the output of the predryer 20 (This will be maintained constant as explained above.). Additionally, a signal T on lead 43, proportional to the temperature of the web at the output of the fourth dryer 24 and a signal V on lead 42 proportional to strip velocity are applied to the computer 34. The signals T and T are derived from temperature measuring instruments 44 and 46, respectively; while the signal V on lead 42 is derived from a tachometer generator 48 connected to one of the heated rolls 26. The web 50, after passing to the sizing rolls 32, passes beneath a beta gauge 52 which will produce an electrical signal D on lead 54 proportional to the density of the issuing web. This signal is used in combination with the velocity signal V on lead 42 to determine the production rate and caliper of the issuing paper.
The outputs of the computer include a signal on lead 56 connected to a motor control circuit 58 for drive motor 60 which, in turn, is connected to a drum 26. As will be understood, a plurality of drive motors will be used in a complete dryer installation and controlled in synchronism by a single motor control circuit. However, only one of such motors 60 is shown herein for purposes of simplicity. An additional output of the computer is a signal on lead 62 which is connected to a steam control valve means 64 for controlling valve 68 which varies the input of steam to the drying rolls of the fourth dryer 24. A signal on lead 63 connected to steam valve control means for the second-third dryer 22 functions in a similar manner. In this regard, the admission of steam to the drums in dryers 22 and 24 varies the temperature at their surfaces. The third or last output from the computer is a signal on lead 70 applied to a slice control mechanism 72 for controlling the position of slice 14. This slice control mechanism 72 also acts to control the speed of wire 16 via drive motor 74 since, as was explained above, the thickness of the web or pulp mat is a function not only of slice gap opening but also of the speed of the wire 16.
Thus, the portion of the computer with which the present invention is concerned controls the caliper of the paper via a signal on lead 70; it controls the temperature of the second-third and fourth dryers via a signal on lead 62; and it controls the speed of the rolls and, hence, the production rate of the drying machine via a signal on lead 56.
With reference now to FIG. 2, there is shown a plot of caliper or thickness of the paper web versus machine speed. Not that there is an envelope drawn around the range of speeds for different calipers over most of the range, that is, between about 10 points and 30 points. Just why the envelope occurs is difficult to say. The top or upper line 76 could be due to the effect of centrifugal force on the sheet as it travels around the drying rolls. That is, above a certain speed the sheet may tend to leave the rolls. Therefore, the drying effect becomes less and limits the maximum speed and production rate. Note that as the caliper and weight of the sheet increases, the maximum speed decreases, verifying the centrifugal force theory. The lower line 78 of the envelope is more difficult to understand since it would appear logical that the drying would be very efficient at lower speeds. However, the fact remains that there is a lower limiting speed, below which drying is ineffective.
In FIG. 3, there are plotted, with reference to caliper, the most frequent operating conditions including steam pressure in the fourth dryer, steam pressure in the second-third dryer, the fourth dryer steam temperature, total steam flow to the machine, and pounds of steam per ton of paper produced. It is significant that the data for pounds of steam per ton of paper lies on a straight line which falls very slightly with increasing caliper. The slope could be explained by the larger contact time (i.e., lower machine speed) associated with the larger calipers. The closeness of this correlation indicates a high degree of consistency in machine operation. Improvement in steam consumption will result in a general lowering of this curve together with perhaps an increase in its slope.
It will be noted that the second-third dryer steam pressure, also its temperature, is always less than the pressure and temperature in the fourth dryer. If T and T, are, respectively, the steam temperatures in the fourth and second-third dryers and z is the caliper, a close linear correlation is:
T s. FJTZZQQTU where:
a and b are empirically derived coefficients which might be termed the difference factor and distribution factor, respectively.
Since there is a well-defined relationship between T, and T a relationship between T production rate and caliper can be derived. In accordance with the present invention, the sheet temperature is measured at the exit from the pre-dryer, T together with the sheet temperature at the size press, T Because of the appearance of a recognizable relationship between T and T a model is based on T alone and the whole dryer system regarded as an elongated heat exchanger. By applying heat exchange concepts in simplified form, and assuming that the production rate in tons per hour is F, we find that:
Heat absorbed by sheet varies as a function of (3), a gross relationship is of the Equating (2) and form:
F 1 g, 34 2)/(T84 T3) K (4) In applying this relationship to the data, the factor K varied inversely as a function of caliper. That is, the heat transfer coefficient between steam and paper decreases as the caliper increases. Alternatively, it can be said that it is much harder to transfer heat to the same weight of paper as the caliper increases. This conforms to experience and results partly from a reduction in surface area per pound in contact with the cylinders, coupled with the greater thickness which causes-the center of the sheet taking longer to heat.
One correlation for the factor K is shown in FIG. 4 where T is assumed constant at 160F and T constant at 240F, giving rise to the empirical relationship:
F log (T T )/(T,., T 29.5 5 log 5 In the control system described herein, the pre-dryer exit temperature T is controlled constant at around l60F and the temperature T is held for a particular grade at a known temperature. The comparatively small variations in T and T which will be experienced in practice will not be sufficient to destroy the value of this empirical relationship as a feed-forward signal.
If we assume that T T and T are constant as described above, Equation (5) can be re-arranged as follows:
C is a constant based upon the fact that all temperatures are assumed constant. Therefore, from the production rate F, either desired or measured on the line, and caliper t the present value of C can be computed and, knowing T and T the predicted value of T can be computed from:
The factor C will be modified by the actual measured value of T with proportional plus reset action. This is a feed-back term and will cause the predicted value of T to be modified so as to hold moisture content constant at the output of the drying machine. T, is computed from T in accordance with Equation (1) given above. The distribution factor b can be modified if desired from a calculated error between actual and desired distribution. Distribution is defined as the ratio of heat in the condensate from the second-third dryer, divided by the total heat in the condensate coming from the second-third dryer and fourth dryer combined.
As was explained above, one of the principal problems associated with the philosophy of grade change control is how to reconcile the slow response of the dryers with the much faster response of both the wet end system and the mechanical portions of the machine. In accordance with the present invention, grade changes are made by divorcing the response of the dryers from the response of the mechanical parts of the machine such that the grade can be changed quickly from one caliper to another even if the production suffers temporaily. The grade changes are thus made at constant steam pressure into the dryers. A subsequent change in the production rate can be carried out at leisure and at a speed commensurate with the response of the dryers. The fact that steam temperature and pres sure are being held constant during the first part of the change does not mean that steam flow will also remain constant during this period. It will, in fact, vary automatically if the heat transfer rate changes, an increase in condensation causing a drop in pressure, which will cause the steam valve to open automatically. Since, as seen from Equation (6) above, all temperatures are maintained constant, a transfer path from one grade to another can be defined by:
(1/F)(29.5-5 log t)=C 3 Assuming that it is desired to change from one caliper and production rate to another, the present value of C is computed from Equation (6). Assuming that C is now held constant, the value of F for the caliper t can be calculated and the production rate adjusted to suit. After a short pause, F for caliper t will be computed and the machine moved to these conditions, and so on until the required change has been completed with the new caliper being completed. A typical family of such transfer paths is shown in FIG. 5 where speed is plotted versus caliper and in FIG. 6 where production rate is plotted versus caliper.
With reference to FIG. 5, let us assume that it is desired to change the caliper from 14 points to 18 points. Under these circumstances, the calculated value for C is 0.636. In the transfer process, the production rate will move progressively downwardly in steps from point 80, for example, along the curve 82 to point 84 while maintaining the value of C at 0.636. Once point 84 is reached, however, the machine speed and production rate can be increased at a caliper of 18 points by increasing the steam temperature into the drying drums; whereupon the operating point may move along the broken line 86 to point 88 which is the new operating point. The transition from point to point 84, of
course, will occur rapidly since it depends only upon the speed of the machine; whereas the transition from point 84 to point 88 will occur slowly due to the inherent thermal lag in the system.
A flow diagram of the computer control system of the invention is shown in FIG. 7. The density D measured by the beta gauge 52 is converted at block 90 into production rate (i.e., tons per hour) from the equation:
K a constant; D density;
W width of the strip; and V velocity as measured by tachometer generator 48. At the same time, the density D is converted into caliper at block 92 in accordance with the equation:
I Basis Weight/D The basis weight is a known factor.
The program starts with entering at 94 the desired caliper 1* and the desired production rate F. As was explained above, this is entered by an operator. The present value ofC is then calculated from Equation (8) above atblock 96 and the program checks at 98 to determine if the present caliper t is equal to desired caliper r*. This is done every 2 minutes; and if the answer is NO, the caliper is changed at 100 in two-point increments. After each two-point increment, a new production rate F is computed at 102 from Equation (8) given above and this, in turn, is converted into speed at 104 in accordance with the equation:
V 1790 (F/B where:
B is the basis weight in pounds per thousand square feet.
A signal proportional to the new speed is then applied via lead 56 to the motor control circuit 58 for motor 60 tovary the speed of the drying drums, either upwardly or downwardly, depending upon whether the caliper is being increased or decreased. Each time the caliper is changed at 100, a signal on lead 70 is applied to the slice control circuit 72 to vary the slice gap opening and the speed of motor 74 (FIG. 1) and, hence, vary caliper. After an elapse of 2 minutes, the above process is repeated; and if actual caliper is not equal to desired caliper at 98, the foregoing process is repeated with the same value of C, the new production rate being calculated at 102 and the new machine speed being calculated at 104. This process continues until I t*; whereupon the program is completed.
Now that t is equal to t*, a YES signal from block 98 triggers block 106 to determine if F is equal to F*. If it is, the YES" signal terminates the program as at 108. This would occur where, with reference to FIG. 5, for example, the production rate is dictated by point 84 at C value of 0.636. However, if the answer is NO, the new program is initiated wherein the speed or velocity is increased in steps at 110, causing the motor control circuit 58 to increase the speed of motor 60. After each increase in speed, a new value of T is computed at block 1 11 from Equation (7) given above until the actual production rate F equals the desired production rate F whereupon the program terminates. At the same time, the value for T is calculated from Equation (1) given above at 114 and this used to produce a signal on lead 63 for controlling valve 66 through steam valve control 65 shown in FIG. 1. At the same time, the calculation of T produces a signal on lead 62. This signal is compared with a signal at a summation point 116 which is the integral of the comparison of actual exit temperature T with desired exit temperature T as integrated in integrator 118. This is the feedback signal as mentioned above. The resulting signal is then applied to the steam valve control 64 for the valve 68.
Although the invention has been shown in connection with a certain specific embodiment, it will be read- .ily apparent to those skilled in the art that-various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention.
We claim as our invention:
1. In the method for controlling a paper drying machine wherein a pulp web is passed through a pre-dryer and thence over first and second pluralities of rolls heated by steam; the steps of generating a feed-forward signal which varies as a function of the temperature of the rolls in said second plurality of rolls from a consideration of the temperature of the web at the output of the pre-dryer, the temperature of the web at the output of said second plurality of rolls, the caliper of the dried web and the production rate of the drying machine; generating a feedback signal which varies as a function of the moisture content of the web leaving the drying machine; modifying said feed-forward signal in response to changes in said feedback signal to develop an error signal; and controlling the admission of steam to i said second plurality of rolls and hence the temperature of the rolls in said second plurality in response to variations in said error signal.
2. The method of claim 1 including the step of controlling the temperature of the rolls in both said first and second plurality of rolls in response to variations in said feed-forward signal.
3. The process of claim 2 including the step of maintaining the temperature at the output of said pre-dryer constant.
4. The method of claim 1 wherein said feedback signal varies as a function of the temperature of the web at the output of said second plurality of rolls and including the step of comparing said feedback signal with a signal proportional to the desired temperature of the web at the output of said second plurality of rolls to develop a modifying signal for modifying said feedback signal before modifying said feedforward signal.
5. The method of claim 1 wherein said feedforward signal is proportional to:
where T is the temperature of the web at the output of said second plurality of rolls, T is the temperature of the web at the output of said pre-dryer, and C is proportional to:
(1/F)(k1 2 ge where F is the production rate of the drying machine, 2 is the caliper of the web leaving the drying machine and k and k are constants.
6. In the method for effecting a change in caliper in a paper drying machine of the type wherein a pulp web is passed through a pre-dryer and thence over a plurality of heated rolls, the steps of running said machine at a first predetermined caliper and production rate, maintaining the temperature of said pre-dryer essentially constant, calculating a transfer constant based upon said first production rate and caliper, thereafter changing the caliper of said web in increments until the caliper matches a new desired caliper which is different than said first caliper, calculating from a consideration of said transfer constant the new production rate after each incremental change in caliper, and varying the speed at which said web passes through said drying machine each time a new production rate is calculated.
7. The method of claim 6 wherein said transfer constant is proportional to:
(l/F) (k k log. t)
where F is the production rate, 1 is the caliper and k and k are constants.
8. The method of claim 6 wherein said caliper is controlled by the position of a slice in a head box and the speed of a wire onto which pulp issues from a slice gap opening, and including the step of controlling the position of said slice to vary the dimensions of said slice gap opening to change said caliper while controlling the speed of said wire in response to a change in caliper to change said production rate.
9. The method of claim 6 including the steps of measuring the caliper of paper issuing from said drying machine, and changing the temperature of said drying rolls after the measured caliper matches said new desired caliper.